Energy harvesters are devices that generally extract a fraction of energy flowing as a result of other systems' or environments' operational conditions. For example, a photovoltaic harvester converts solar radiation to electrical current that can be used to operate a sensor. Typical sources of energy flow include mechanical straining, fluid movement, electromagnetic radiation (solar and other RF), thermal gradients, and vibration. Energy harvesters use a transducer to convert an energy flow into electrical power that can be stored or applied to an electrical load. A circuit compliments the transducer for optimally extracting electrical energy form the transducer, storing that energy, and delivering it to the load at a fixed voltage under varying loads.
Prior art in the general field of energy harvesting encompasses a variety of different transducers that can be used to convert the energy flow into electrical power. These transducers range from electromagnetic generators driven by mechanical hand cranks (U.S. Pat. No. 4,360,860) to piezoelectric transducers used to extracted energy from varying centripetal force (U.S. Pat. No. 3,463,942) to antennas used to harness electromagnetic energy in the radio frequency band (U.S. Pat. No. 6,882,128). Novel materials such as electrostrictive polymers are new enabling energy harvesting transducers that expand the range of potential applications (U.S. Pat. No. 6,433,465).
Some of the most relevant prior art in the field of energy harvesting describe the method or mechanism to impedance match the energy flow to the transducer. For example a Helmholtz resonator can be used in an acoustic energy harvester to amplify force applied to a piezoelectric harvester (U.S. Pat. No. 7,116,036), a resonant circuit can be coupled to an antenna for enhancing RF harvesting (U.S. Pat. No. 6,856,291), and a force amplification mechanism that matches a typical compressive load to the unusual high load, but low displacement actuation properties of piezoelectric materials (U.S. Pat. No. 7,446,459).
Other prior art includes specific application of energy harvesting to particular devices and systems. Small scale applications include solar, thermal, and vibration harvesting for powering time pieces (U.S. Pat. No. 4,320,477) and miniature wireless switches for vehicle door locks (U.S. Pat. No. 6,933,655). Larger scale applications include piezoelectric elements in a tire tread for vehicle health monitoring (U.S. Pat. No. 4,504,761) and harvesters for extracting energy from wave motion to power electronics on buoys (U.S. Pat. No. 4,631,921).
The invention presented herein improves upon the prior art in the particular field of piezoelectric vibration energy harvesting. Vibration energy harvesters are generally dynamic systems that have an inertial proof mass suspended by a spring connecting the mass to a mounting base. The mounting base is generally rigidly attached to a host structure. Vibration of the host structure is resisted by the inertial proof mass causing deformation of the spring. In the case of piezoelectric vibration harvesters, the spring is instrumented with piezoelectric materials so that spring deformation strains the piezoelectric material which in turn generates electrical charge. The electrical charge can then be extracted from the piezoelectric element and stored.
In many applications the host structure has a periodic and fixed frequency feature of its vibration that enables tailoring the harvester to operate optimally for one particular condition. In particular, the spring and mass can be adjusted so that they resonate at the fixed input frequency.
Harvester cost and power density (power output per unit mass) are the two most important metrics that define their value for most practical applications. The cost is typically a function of the fabrication methods and device size scale. The power density is dictated by the following expression:
            P      _        =                            m          p                ⁢                  δ          e                ⁢                  V          2                ⁢        ω                    4        ⁢                              (                                          δ                e                            +                              δ                m                                      )                    2                ⁢                  (                                    m              p                        +                          m              s                                )                      ,where the proof mass is mp, the supporting structure for the proof mass is ms, the base vibration velocity amplitude is V, the vibration frequency is ω, δe is the electrical damping caused by energy harvester transducer, and δm is the parasitic mechanical damping.
The frequency and vibration amplitude are generally fixed by the application. The proof mass is typically much larger than the structure mass and so the power density generally does not depended on mass the harvester. The remaining parameters that strongly influence the power density are the mechanical and electrical damping. According to the power density expression, for a given mechanical damping there will be an optimal electrical damping. However, higher mechanical damping always diminishes the power density and since it is often greater than the electrical damping, it becomes the critical parameter for determining a harvester's power density.
The parasitic damping is primarily dictated by the device structural design and material section. Interfaces between discrete component such as fasteners, bonded joints, and press fits, are the primary source of parasitic damping.
Although prior art has not explicitly addressed power harvester design from this perspective, some inventions have features that indirectly minimize this issue for MEMS (Microelectromechanical system) scale devices. U.S. Pat. No. 6,858,970 shows a plurality of beams, masses, and piezoelectric members disposed in one assembly. U.S. Pat. No. 6,938,311 describes a similar multi mass system with additional features such as a sheath to hold the masses and a deflection limiter. The multi-beam system minimizes parasitic damping by using a single substrate for all the beams. This single substrate monolithic construction is typical of all MEMS devices that use IC processes for fabrication. The circuitry for the energy extraction and condition can also be included on a single monolithic chip (U.S. Pat. No. 6,407,484). However, MEMS devices have limited applicability in real applications due to robustness, limited power density, deposition of piezoelectric thin films, and squeezes film damping.
A secondary, but similarly important, drawback of the interface between discrete components is variability in the system stiffness. Variability in stiffness is important because the harvester's resonance frequency (primarily dictated by its proof mass and spring stiffness) is designed to match a host structure's fixed base excitation vibration frequency. A miss match due to variation in the component interfaces strongly influences the device performance. In addition, component interfaces tend to change over time in response to periodic mechanical strain which in turn causes the harvester frequency to drift.
Prior art describing frequency tuning includes a mechanism in U.S. Pat. No. 7,521,841 and a movable fluid bead in U.S. Pat. No. 7,293,411 that adjusts the system's stiffness and mass.
The harvester designs in the present state of the art do not sufficiently address the critical issues of component interfaces and associate frequency drift for devices that are fabricated on a macro size scale. A novel solution to these issues is presented in the following invention.